Why is it that when secretory rate increases in the pancreas the concentration of chloride ions decreases, and the concentration of bicarbonate increases in the production of isotonic NaCl secretion?

The modification that goes on in the duct of the acinus is dependent on flow. If the flow is faster, then less ions can be reabsorbed from the primary secretion. In the pancreas, chloride ions are removed, so surely their concentration should go up as the flow increases?

Bicarbonate on the other hand is added to the primary secretion in the duct. So shouldn't its concentration in the secretion stay the same or even increase as flow increases?

This is an approximation of the graph given in our lecture notes; from Boron & Boulpaep, Medical Physiology, 1st Ed, p919 - Sorry for the bad quality reconstruction
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Will PerryApr 24 '13 at 23:50

" In the pancreas, chloride ions are removed, so surely their concentration should go down as the flow increases?" According to the logic of your question I think this should be "should go up".
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Alan BoydApr 25 '13 at 5:20

1 Answer
1

I am attaching the picture from Boron & Boulpaep, Medical Physiology, 2nd Ed, p921 here to clarify the details left out in the hand drawn picture.

The flow of the digestive pancreatic enzymes increase mainly during food consumption. This is brought about by a number of redundant systems. The fasting secretion of pancreatic enzymes depends on the MMC (migrating motor complex).

Bicarbonate ions are exchanged with chlorine ions in the apical membrane of the acinar cells through a Chloride Bicarbonate Exchanger. This process is driven by:

Secretin - most important humoral factor - Increases secretion

ACh through muscarinic receptors - Increases secretion

GRP (Gastrin Releasing Peptide) - Increases secretion

Substance P - Decreases secretion

Mechanism of action of secretin seems to be through activation of cAMP. However even small amounts of secretin which do not raise the cAMP levels measurably can stimulate bicarbonate secretion. This suggests secretin response may be by

Unmeasurably small amount of increase in cAMP

Raise in cAMP levels in localised compartments

Activation of alternative second messenger pathways

In case of GRP the second messenger is not known.

The answer to your question is that when secretin is secreted, the Bicarbonate-Chloride transporter is accelerated to achieve the result displayed. This mainly happens when food enters the small intestine so as to neutralize the acid permitting the digestive enzymes to function at the optimal pH.

The Chloride-Bicarobonate Transporter is an exchange transporter which explains why there is a sharp fall in the chloride levels.